1. Field of the Invention
The present invention relates generally to a transmission/reception apparatus and method in an OFDM (Orthogonal Frequency Division Multiplexing) mobile communication system, and in particular, to an apparatus and method for reducing a high peak-to-average power ratio (PAPR) caused by multiple subcarriers.
2. Description of the Related Art
Generally, in an OFDM (Orthogonal Frequency Division Multiplexing) technology, spectrums of subchannels overlap with one another while maintaining orthogonality, increasing spectrum efficiency. In addition, modulation is realized by inverse fast Fourier transform (IFFT), and demodulation is realized by fast Fourier transform (FFT).
A brief description will now be made of operation of a transmitter and a receiver for a mobile communication system supporting the OFDM.
In an OFDM transmitter, input data is modulated with subcarriers through scrambler, encoder, and interleaver. The transmitter provides a variable data rate, and has different code rates, interleaving sizes and modulation schemes according to the data rate. Commonly, the encoder uses a code rate of ½ or ¾, and in order to prevent burst errors, a size of the interleaver is determined based on the number (NCBPS) of coded bits mapped per OFDM symbol. The transmitter uses a modulation scheme of QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary Phase Shift Keying), 16QAM (16-ary Quadrature Amplitude Modulation) or 64QAM (64-ary Quadrature Amplitude Modulation) according to the data rate. A predetermined number of pilots are added to the signals modulated with a predetermined number of subcarriers, and the pilot-added modulated signals constitute one OFDM symbol after passing through an IFFT block. A guard period for eliminating inter-symbol interference in a multipath channel environment is inserted into the OFDM symbol, and the guard period-inserted OFDM symbol is finally applied to an RF (Radio Frequency) block through a symbol waveform generator before being transmitted over a channel.
In a receiver, a reverse operation of the transmitter is performed and a synchronization process is added. First, a process of estimating a frequency offset and a symbol offset using a predetermined training symbol must precede. Thereafter, data symbols, from which the guard period is removed, are restored with a predetermined number of subcarriers into restored signals to which a predetermined number of pilots are added, after passing through an FFT block. An equalizer for coping with a path delay phenomenon removes signal distortions caused by a channel from a received signal by estimating a channel condition. Data channel-response-compensated by the equalizer is converted into a bit stream, deinterleaved by a deinterleaver, and output as final data through an encoder for error correction and a descrambler.
The OFDM can realize efficient digital modulator/demodulator, and is less susceptible to frequency-selective fading or narrow-band interference. Therefore, the OFDM has been selected as a standard of the European digital broadcasting system and standards of the high-capacity mobile communication system, such as IEEE 802.11a, IEEE 802.16a and IEEE 802.16b. That is, the OFDM is a technique efficient to high-speed data transmission.
However, since a mobile communication system supporting the OFDM (hereinafter, referred to as “OFDM mobile communication system”) transmits data with multiple subcarriers, an amplitude of a final OFDM signal can be expressed as the sum of amplitudes of the multiple subcarriers. In particular, the multiple subcarriers, if they have the same phases, have a very high peak-to-average power ratio (PAPR). In this case, the PAPR deviates from a linear operation range of an amplifier, so an output signal of the amplifier has a distortion.
In the OFDM mobile communication system, amplitude of an OFDM signal is variable according to a phase difference of the subcarriers. In addition, a back-off phenomenon where an operation point is separated far away from the maximum output point of an amplifier occurs, reducing efficiency of the amplifier and increasing power consumption. Such a signal with a high PAPR decreases efficiency of a linear amplifier, and in the case of a nonlinear amplifier, the signal shifts an operation point to a nonlinear region, causing mutual modulation between carriers and spectral regrowth to an adjacent frequency band.
Therefore, the OFDM mobile communication system needs a method for decreasing the PAPR. A typical method for reducing the PAPR is to expand a linear region of a power amplifier in order to prevent signal distortion, so that every signal can be linearly amplified. An alternative method is to induce a back-off phenomenon of a nonlinear amplifier so that the nonlinear amplifier can operate in a linear region. However, the two methods both cause a reduction in power efficiency and an increase in cost.